Friday

August 1, 2014

August 1, 2014

Posted by **jeni** on Tuesday, May 8, 2012 at 11:01pm.

PART A: What is the minimum value of needed to move the two blocks?

PART B: If the force is 10% greater than your answer for (a), what is the acceleration of each block?

- physics- please ! -
**Elena**, Wednesday, May 9, 2012 at 2:12pmThe forces acting on the blocks are:

top block (m1) :

Gravity m1•g (downwards), the normal force on contact between two blocks N1 (upwards), friction force between two blocks F1(fr) ( to the right), Tension T (to the left).

Bottom block (m2).

Gravity m2•g (downwards), gravity m1•g (downwards), the normal force on contact between blocks and the surface N2 (upwards), friction force between two blocks F1(fr) (to the left), friction force between the block and the surface F2(fr) (to the left), tension T (to the left) , unknown force F (to the right).

Equations of the blocks motion (projections on the horizontal and vertical axes):

top block -

x: m1•a = T – F1(fr),

y: 0 = – m1•g + N1. => N1 = m1•g

bottom block –

x: m2•a = F –T –F1(fr) – F2(fr),

y: 0 = - m1•g – m2•g +N2. => N2 = (m1+m2) •g

(a) If the two blocks are just to move, then the force of static friction will be at its maximum, and sothe frictions forces are as follows.

F1(fr) = k1• N1 = k1•m1•g,

F2(fr) = k1• N2 = k1•(m1+m2)•g,

a = 0;

0 = T – F1(fr), => T = F1(fr),

0 = F –T –F1(fr) – F2(fr),

Solve for F.

F = T +F1(fr) + F2(fr) = F1(fr) +F1(fr) + F2(fr) = 2 •F1(fr) + F2(fr) =

= 2• k1•m1•g + k1•(m1+m2)•g = k1•g (3•m1+m2) = 0.65•9.8•(3•3+5) = 89.18 N.

(b) Now force is F1 = 1.1•89.18 = 98.1 N

m1•a = T – F1(fr),

m2•a = F1 –T –F1(fr) – F2(fr),

(m1+m2) •a = F1 - 2 •F1(fr) - F2(fr) =F1 –k2•g•(3•m1+m2),

a = {F1 –k2•g•(3•m1+m2)}/ (m1+m2) =

={98.1 – 0.42•9.8•(3•3+5)}/(3+5) = 5.06 m/s².

**Related Questions**

physics- please ! - A 3.0kg- block sits on top of a 5.0kg block which is on a ...

physics - A 3.0kg- block sits on top of a 5.0kg block which is on a horizontal ...

physics - Two blocks are tied together with a string as shown in the diagram.(...

Forces w/ tension - Assume that three blocks portrayed below move on a ...

Math Physics - Two blocks of masses 40.0kg and 20.0kg are stacked on a table ...

physics - I would like to thoroughly understand each steo to solve this. In 0....

Physics - Two blocks are tied together with a string as shown in the diagram.(...

Physics - Two blocks of masses 40.0kg and 20.0kg are stacked on a table with the...

Physics - Three blocks on a frictionless horizontal surface are in contact with ...

Physics - A block of mass 3m is placed on a frictionless horizontal surface, and...